Marine propellers are sometimes constructed by fastening individually fabricated blades to a hub. This is always the case with controllable pitch propellers (CP propellers) and can also be the case with propellers having fixed blades. The term "built-up marine propeller" will be used herein to refer to both CP and fixed-blade marine propellers in which individual blades are fabricated separately and are fastened to a hub.
Like any rotating device, it is important that a marine propeller be dynamically balanced to minimize vibration and thereby prevent undesirable loadings of the propeller components, the ship's shafting, and the shaft bearings and their mounts, and minimize noise. At one time, the blades of built-up marine propellers were hand-finished and were balanced against a standard blade weight by removing material from the blade surfaces. The resulting blades often deviated somewhat from the desired profile, which resulted in small, but nonetheless undesirable, differences in hydrodynamic performance among the blades.
With the advent of computer numerically controlled (CNC) 5-axis milling machines, it became possible and highly advantageous to finish the blades with such machines, which enables the blade surfaces to be highly accurate. CNC machine-finished blades, accordingly, require relatively small balance corrections, and such corrections have heretofore been made by providing a cavity on the side of the blade flange that faces the propeller hub and installing in the cavity lead balance members specially fabricated to a have a weight that is determined from a spin-balancing test, i.e., a test in which a simulated propeller consisting of a dummy hub and a set of blades installed on the dummy hub in a configuration corresponding to the actual propeller in which the blade will be used is spun (rotated) and the parameters (direction and magnitude) of the imbalance are determined. The locations and the amounts of the lead balance members required to balance the simulated propeller are calculated from the test results. The balance members are fabricated and installed on the blade flanges as required, which involves removing the blades from the dummy hub. The blades are reinstalled, the spin-balancing test is repeated, and any further balance adjustments are made by altering the balance members.
The effect on the balance of the propeller of installing balance members on the propeller blades is, of course, a function of the masses of the balance members and the distances of the balance members from the axis of rotation of the propeller. When balance members are installed in the blade flanges, the distance from the propeller axis is relatively small, and balance members with relatively large masses (high weights) are required. To minimize the masses of the balance members, it has been conventional to balance the propeller blades in sets and to install them as sets on the propeller. For both balancing and installation, the blades are arranged in a sequence, based on their weights, that will make the propeller most closely balanced before balance members are added and minimize the sizes and the number of balance members required to achieve balance. An important disadvantage of balancing the blades in sets is that they have to be used in sets. If one or more, but less than all, of the blades of a propeller have to be changed for some reason, it is necessary to change the whole set.